1. Field of the Invention
The invention relates to a process for preparing optionally substituted diphenylamines by reacting the corresponding cyclohexanones and anilines in the presence of a supported catalyst containing one or more metals of the type described below having a dehydrogenating action.
2. Description of the Related Art
A process for preparing diphenylamine and its derivatives is known from DE-OS (German Published Specification) 2,331,878, in which imines such as N-cyclohexylidene-aniline and its derivatives are used as starting materials and are dehydrogenated in the gaseous phase in the presence of supported catalysts based on nickel, palladium, or copper-chromium.
Furthermore, it is known from DE-OS (German Published Specification) 2,520,893 to prepare diphenylamine by catalytic dehydrogenation of compounds and/or mixtures of compounds comprising wholly or partly hydrogenated diphenylamine, in the presence of a dehydrogenation catalyst containing nickel-chromium, aluminium, copper, manganese and alkali metal. Binuclear aromatic imines are mentioned as such compounds in the examples of implementation.
According to DE-OS (German Published Specification) 1,493,942 the preparation of diphenylamine from cyclohexanone and aniline is carried out in the liquid phase via the intermediate stage of the associated Schiff base in the presence of a dehydrogenation catalyst and a hydrogen acceptor. Such a process is too costly for technical implementation.
Also, the dehydrogenation of dicyclohexylamine to diphenylamine over noble metal catalysts has already been proposed (DE-OS (German Published Specification) 3,801,754).
The aforementioned processes require starting materials that are relatively costly to prepare, such as N-cyclohexylidene-aniline or dicyclohexylamine, and require improvement for a process to be performed industrially.
A process for preparing diphenylamine is proposed in DE-OS (German Published Specification) 2,249,089, in which aniline vapour is passed over an aluminum oxide catalyst that is or has been treated with boron trifluoride. However, the space-time yields that can be achieved are low for industrial production of diphenylamine.
According to German Patent Specification 530,736 aniline and phenol are reacted at elevated temperature in the presence of catalysts (kaolin or bleaching earth) treated with acids. Although the reaction is carried out with an excess of aniline, some of the aniline is lost.